Imaging sensors are used to capture visible light or other types of incident radiation emissions and produce an image in a variety of applications. Many parameters of an imaging sensor are temperature dependent. It is therefore desirable to be able to determine image sensor temperature and use the temperature data in a variety of ways including compensation for the effects of temperature.
For example, in CMOS active pixel sensors (APS), overall analog power consumption is primarily the sum of the bias currents of each of the individual analog circuits. These bias currents are proportional to the master current Iref, generated by a dedicated analog cell commonly referred to as the master current reference. In many cases, the master current reference cell has a positive thermal coefficient (PFAT) proportional to changes in absolute temperature, that is, as temperature increases, the master current increases, leading to a higher power consumption. On the other hand, as the temperature decreases, the reference current decreases leading to a reduction of the bias current for the analog circuits. With respect to its room temperature nominal value and within the temperature range of −20° C. to +65° C., it has been observed that the master current linearly varies from −15% to +15% from a nominal value. Furthermore, at room temperature the distribution of master current values in a population of CMOS chips is gaussian shaped with a standard deviation (sigma) of ˜7% of the mean value. Chip-to-chip variations in master current values originate from manufacturing process variations and can typically only be corrected by individual trimming of the master current reference cell output. When process and temperature variations are combined together, temperature drift or changes cause variations of the master current Iref causing erroneous operation or possibly chip failure if the chip temperature changes too much.
Contemporary CMOS imager chips lack an optimal system for automatically compensating for variations in temperature in large measure because sensing of imager temperature requires dedicated circuitry which adds to design cost. Thus, a new approach is needed to provide a simplified temperature sensing system for CMOS imaging operations.